A New Approach to Stimulate Population Proliferation ofTrichodermaspecies and Other Potential Biocontrol Fungi Introduced into Natural Soils

Abstract

Population densities of isolates of T. viride (T-1-R4) and T. harzianum (WT-6-24) increased about 104- and 103-fold, respectively, in natural soil during the first 3 wk of incubation when the antagonists were added as a mycelial preparation (sterile bran-sand-water, [1:1:2,wt/wt/vol] inoculated with conidia and allowed to incubate 1-3 days before addition to soil). The preparation applied consisted of 1.0% bran and 103-104 young, actively growing propagules per gram of soil. Populations did not increase when conidia were added to soil with or without bran. An 8-day-old preparation was less effective in stimulating a population increase and a 40-day-old preparation was entirely ineffective. The number of colony forming units (cfu) gradually decreased over 18-36 wk and stabilized at 105-106/g of soil. Addition to soil of a mycelial preparation to provide as little as 0.01% bran and 101-102 propagules per gram resulted in a 105- to 106-fold increase in population. Proliferation in soil resulted only when hyphae were in intimate contact with or in possession of the substrate (bran). Mycelium of T-1-R4 and WT-6-24 in bran resulted in population densities of about 108/g of soil in several soil types (sandy loam, loamy sand, loam, silty clay loam) of various organic matter content. Proliferation of populations of Trichoderma in soil as a result of addition of mycelial preparations was accompanied by an increase in metabolic activity as determined by CO2 evolution and by growth of the antagonist from a food base to pieces of organic matter in soil. A wide variety of isolates of T. viride, T. harzianum, and T. hamatum as well as other antagonists (Talaromyces flavus, Gliocladium virens, G. roseum, G. catenulatum and Aspergillus ochraceus) added to soil as mycelial preparations gave results similar to those obtained with T-1-R4 and WT-6-24. Population densities increased up to 106-fold within the first 3 wk of incubation, with the rate of increase greatest in the 1st wk.